Are you serious with that question?
You break the damned law.
"Ah, geez whiz guys. I really wish I could help save the universe but you know the accords just won't let me. I'm really sorry. It's past my bedtime."
I'm not going to fight for people who want to kill me and bury me in the ocean.
Not worth risking getting killed by military goons, being shot in the back while fighting Thanos.